A neurological and electroencephalographic (EEG) prospective study was performed in 34 leukemic patients receiving busulfan (BU) plus cyclophosphamide (CY) before bone marrow autologous transplant. During BU treatment and briefly thereafter, all patients were given anticonvulsant prophylaxis with phenobarbital. Neurological evaluations were performed daily and EEGs, recorded one week before and soon after the end of the BU regimen, were re-evaluated two months later. Basal EEGs were normal in 23 of the 34 patients, focal abnormalities were detected in 9 cases, and generalized epileptic discharges were present in one (one subject was not studied). EEGs performed at the end of BU administration showed generalized spike/polyspike-and-wave discharges in 21 of the 34 patients, with associated myoclonic epilepsy in 10 subjects appearing on the 3rd or 4th day of treatment. Focal abnormalities were present in 6 patients and constant EEG normality in 7. It is known that myoclonic epilepsy can be induced by drugs. Oral administration of high-dose BU is followed by a high-dose cerebrospinal fluid concentration of the drug. Therefore, myoclonic epilepsy and/or paroxysmal epileptiform EEG discharges may be observed in leukemic patients undergoing high-dose BU therapy.